Weeeeeeeeell, this is one of those slippery slopes. While I agree that upon first use, I thought they were backwards as well, you'll find that every other similar UI control in this game.

Selling an item? Pushing all the way to the left maxes the number out, pushing to the right minimizes it.
Putting an item up for sale? Pushing all the way to the left maxes the number out, pushing to the right minimizes it.

Unsurprisingly, the control does the same thing.